1. Field of the Invention
The present invention relates to an improvement in the structure of a vibration wave motor driven by a travelling vibration wave.
2. Description of the Prior Art
A vibration wave motor transduces a vibration motion created by application of a periodic voltage to electrostrictive elements to a rotational motion or a linear motion. Because it does not require windings as opposed to a conventional electromagnetic motor, it is simpler and smaller in structure and produces a high torque at a low rotating speed.
FIGS. 1 and 2 show a principle of drive in a prior art vibration wave motor. FIG. 1 illustrates generation of the surface wave in the motor. Numeral 1 denotes a vibration member, and numerals 2a and 2b denote electrostrictive elements which are bonded or welded to the vibration member 1 (usually made of metal) and arranged on one side of the vibration member 1 with a spatial phase difference of .lambda./4 therebetween.
The vibration member 1 is used as one electrode for the electrostrictive elements 2a and 2b and an A.C. voltage V=V.sub.0 sin .omega.t is applied to the electrostrictive element 2a from A.C. power supply 3a while an A.C. voltage V=V.sub.0 sin (.omega.t.+-..pi./2) having a phase difference of .lambda./4 is applied to the electrostrictive element 2b, where signs "+" and "-" are selected by a phase shifter 3b in accordance with a direction of movement of the movable member 5. Let us assume that the sign "-" is selected and the voltage V=V.sub.0 sin (.omega.t-.pi./2) is applied to the electrostrictive element 2b.
When only the electrostrictive element 2a is vibrated by the voltage V=V.sub.0 sin .omega.t, a vibration by a standing wave is generated as shown in FIG. 1(a), and when only the electrostrictive elements 2b is vibrated by the voltage V=V.sub.0 sin (.omega.t-.pi./2), a vibration by a standing wave is generated as shown in FIG. 1(b). When the two A.C. voltages having the phase difference therebetween are simultaneously applied to the electrostrictive elements 2a and 2b, the surface wave travels.
FIGS. 1(A), 1(B), 1(C) and 1(D) show the surface waves at times t=2n.pi./.omega., t=.pi./2.omega.+2n.pi./.omega., t=.pi./.omega.+2n.pi./.omega. and t=3.pi./2.omega.+2n.pi./.omega., respectively, and the wavefront travels in the x-direction.
Such a travelling surface wave includes a longitudinal wave and a lateral wave. Looking at a mass point A of the vibration member 1 as shown in FIG. 2, a longitudinal amplitude u and a lateral amplitude w make a rotating elliptic motion.
A movable member 5 is press-contacted to the surface of the vibration member 1 and it contacts only to an apex of the vibration member. (Actually, it contacts an area having a definite width.) Accordingly, the vibration member 5 is driven by the longitudinal amplitude component u of the elliptic motion of the mass points A, A; . . . at the apex and it moves in an arrow direction N.
When the phase of the voltage is shifted 90.degree. by the 90.degree. phase shifter, the surface wave travels in -x direction and the movable member 5 moves in the opposite direction to the direction N.
A structure of a rotary vibration wave motor which causes a rotational motion by the above vibration wave motor is shown in FIG. 3, and a sectional view thereof is shown in FIG. 4. Numeral 11 denotes an elastic vibration member made of metal; numeral 12 denotes an electrostrictive element coupled to the vibration member 11; numeral 15 denotes a movable member press-contacted to the vibration member 11; numeral 16 denotes a rotary disc (rotary shaft) which is rotated with the movable member 15; numeral 17 denotes a vibration absorber which supports the vibration member 11; and, numeral 18 denotes a stator.
A periodic voltage is applied to an electrostrictive element 12 similar to the electrostrictive element 2 in structure from an external power supply, and a frequency of the periodic voltage is selected to a drive frequency at which the vibration member 11 resonates. Thus, a travelling wave is generated in the vibration member 11, and a frictional force is applied by the travelling wave generated in the vibration member 11 to a movable member 15 which is press-contacted to the vibration member 11 by a spring 20 through a thrust bearing 19 so that the rotary shaft 16 coupled to the movable member 15 is rotated. A fixed cover 21 is fixed to the stator 18 by bolts 22, and the vibration absorber 17 is inserted between the electrostrictive element 12 and the stator 18 to prevent supersonic vibration of the vibration member 11 from propagating to the stator 18.
However, because the vibration member 11 is of ring shape, when this vibration wave motor rotates, the vibration member 11 not only vibrates in a direction normal to the ring plane in the off-plane vibration mode shown in FIG. 2 but also twists in a circumferential direction of the ring as shown in FIG. 5. Thus, both the bending vibration normal to the ring plane and twisting along the circumference of the ring are generated. The vibration wave motor utilizes the travelling wave by the bending vibration generated normally to the ring plane to drive the movable member 15. However, it does not effectively utilize the travelling wave by the twist component of the vibration generated in the circumferential direction of the ring.
Accordingly, the energy of the twist component of the vibration generated in the circumferential direction is lost, and the efficiency of the vibration wave motor is lowered.
As shown in FIG. 5, the amplitude of the twist component of the vibration in the circumferential direction of the ring is larger at a radially outer point than at a radially inner point. Accordingly, a magnitude of the elliptic motion of the mass point on the surface of the vibration member 11 is larger at the radially outer point than at the radially inner point. Since the travelling wave by the bending vibration normal to the ring plane is utilized, the movable member 15 on the vibration member 11 primarily contacts the radially outer area of the ring and does not contact the radially inner area.
Thus, the contact area between the movable member 15 and the vibration member 11 is substantially reduced, the propagation efficiency of the travelling wave of the vibration member 1 to the movable member 15 is lowered and a sufficient output is not produced.